JP2018204549A - Exhaust emission control device for engine - Google Patents

Abstract

To ensure satisfactorily both responsiveness to achieve a target air-to-fuel ratio and a sufficient air volume in a high load region in NOx reduction control.SOLUTION: A NOx catalyst 41 is disposed in an exhaust passage 40 of an engine 1. A change of supercharging pressure is performed by a supercharging control valve 29 disposed in a passage for bypassing an exhaust turbo supercharger 23. To change an air-fuel ratio of exhaust gas to a target air-fuel ratio required for NOx reduction, air volume adjustment is performed by an air volume control valve 25 disposed in an intake passage 20 or by the supercharging control valve 29. The supercharging control valve 29 controls the air volume adjustment after the air volume control valve 25 performs the air volume adjustment or on the condition that the air-fuel ratio converges close to the target air-fuel ratio through the air volume adjustment performed by the air volume control valve 25.SELECTED DRAWING: Figure 5

Description

  The present invention relates to an exhaust emission control device for an engine.

  In the engine, the NOx catalyst disposed in the exhaust passage of the engine occludes NOx in the exhaust gas when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio, and the exhaust gas There are some which reduce the stored NOx when the air-fuel ratio is close to the stoichiometric air-fuel ratio or richer than the stoichiometric air-fuel ratio. Patent Document 1 discloses that when NOx occluded in the NOx catalyst becomes a predetermined value or more, the amount of air is reduced to enrich the air-fuel ratio of the exhaust gas, and NOx reduction control is executed. Yes.

Japanese Patent Laid-Open No. 10-184418

  When executing NOx reduction control (DeNOx control), the air amount is reduced and the fuel post-injection is performed to enrich the air-fuel ratio of the exhaust gas. Then, as a region for executing the NOx reduction control, a predetermined reduction execution region having at least the engine load as a parameter is set.

  In the high load region in the above reduction execution region, a large amount of air is required to suppress the generation of soot and the like. For this reason, the amount of air for achieving the target air-fuel ratio is ensured by adjusting the supercharging state of supercharging. It will be necessary. However, since the air amount adjustment by the supercharging state adjustment has poor responsiveness, when the DeNOx control is started, the responsiveness for obtaining the target air-fuel ratio is deteriorated, and the NOx reducing ability is reduced accordingly. become.

  The present invention has been made in consideration of the above circumstances, and its purpose is to ensure the responsiveness of achieving the target air-fuel ratio and to ensure a sufficient amount of air in a high load region when performing NOx reduction control. It is an object of the present invention to provide an exhaust emission control device for an engine that can satisfy both requirements.

In order to achieve the above object, the following first solution is adopted in the present invention. That is, as described in claim 1,
A fuel injection valve for supplying fuel to the engine;
An air amount control valve for adjusting the amount of air supplied to the engine;
An exhaust turbocharger,
A supercharging control valve for adjusting a supercharging state by the exhaust turbocharger;
NOx in the exhaust gas is occluded when the exhaust gas air-fuel ratio is leaner than the stoichiometric air-fuel ratio, and the exhaust gas air-fuel ratio is near the stoichiometric air-fuel ratio or higher than the stoichiometric air-fuel ratio. A NOx catalyst that reduces the stored NOx when it is rich,
When the NOx reduction condition is satisfied, the NOx reduction control for reducing the NOx stored in the NOx catalyst by setting the air-fuel ratio of the exhaust gas to a target air-fuel ratio capable of reducing the NOx stored in the NOx catalyst is executed. NOx reduction control means for
With
The NOx reduction control means adjusts the air amount by the air amount control valve and the supercharging control valve and sets the air-fuel ratio of the exhaust gas to the target air-fuel ratio, and adjusts the air amount by the air amount control valve Is set to allow the air amount adjustment by the supercharging control valve after the air-fuel ratio of the exhaust gas is converged to the vicinity of the target air-fuel ratio by
It is like that.

  According to the first solution described above, the air amount is adjusted by the air amount control valve having excellent responsiveness before the air amount is adjusted by the supercharge control valve having poor responsiveness. The fuel ratio can be quickly converged to the vicinity of the target air-fuel ratio required for NOx reduction. Then, by adjusting the air amount by the supercharging control valve after convergence to the target air-fuel ratio, a large amount of air can be ensured, and NOx reduction in a high load region can be handled.

A preferred mode based on the first solution is as described in claim 2. That is,
The NOx reduction condition is set when the engine operating state is at least in the reduction execution region set with the engine load as a parameter, and the reduction execution region is divided into a low load region and a high load region in advance. And
The NOx reduction control means adjusts the air amount by the air amount control valve when in the low load region of the reduction execution region, while the engine load is in the high load region of the reduction execution region. Adjusts the air amount by the supercharging control valve to control the air-fuel ratio of the exhaust gas to the target air-fuel ratio, and the air-fuel ratio of the exhaust gas is controlled by the air amount control by the air amount control valve. It is set to allow the air amount adjustment by the supercharging control valve after being converged near the target air-fuel ratio,
It is like that. According to the above solution, in a low load area where a large amount of air is not required, in a high load area while ensuring responsiveness and accuracy for achieving the target air-fuel ratio by adjusting the air quantity by the air quantity control valve. A large amount of air required can be secured. Then, after the air amount adjustment by the air amount control valve converges to the vicinity of the target air-fuel ratio, the air amount adjustment by the supercharging control valve is performed. It is possible to shift to NOx reduction control in a high load region.

In order to achieve the above object, the following second solution is adopted in the present invention. That is, as described in claim 3,
A fuel injection valve for supplying fuel to the engine;
An air amount control valve for adjusting the amount of air supplied to the engine;
An exhaust turbocharger,
A supercharging control valve for adjusting a supercharging state by the exhaust turbocharger;
NOx in the exhaust gas is occluded when the exhaust gas air-fuel ratio is leaner than the stoichiometric air-fuel ratio, and the exhaust gas air-fuel ratio is near the stoichiometric air-fuel ratio or higher than the stoichiometric air-fuel ratio. A NOx catalyst that reduces the stored NOx when it is rich,
When the NOx reduction condition is satisfied, the NOx reduction control for reducing the NOx stored in the NOx catalyst by setting the air-fuel ratio of the exhaust gas to a target air-fuel ratio capable of reducing the NOx stored in the NOx catalyst is executed. NOx reduction control means for
With
The NOx reduction condition is set when the engine operating state is at least in the reduction execution region set with the engine load as a parameter, and the reduction execution region is divided into a low load region and a high load region in advance. And
The NOx reduction control means adjusts the air amount by the air amount control valve when the engine load is in the low load region of the reduction execution region, while the engine load is in the high load region of the reduction execution region. In this case, the air amount adjustment by the supercharging control valve is performed, and the control of the air amount adjustment by the supercharging control valve is performed from the state in which the air amount adjustment by the air amount control valve has been performed. Set to allow at the time of transition,
It is like that. According to the second solution described above, substantially the same effect as in claim 2 can be obtained.

A preferred mode based on the second solution is as described in claim 4. That is,
The NOx reduction control means controls the air amount adjustment by the supercharging control valve on the condition that the air-fuel ratio of the exhaust gas has converged near the target air-fuel ratio by the air amount adjustment by the air amount control valve. It can be set to allow. In this case, after confirming that the air-fuel ratio of the exhaust gas has converged to the vicinity of the target air-fuel ratio, the process proceeds to the air amount adjustment by the supercharging control valve, so the air amount adjustment is performed by the supercharging control valve. It is possible to prevent or suppress the fluctuation of the air-fuel ratio immediately after shifting to the state.

  According to the present invention, when performing NOx reduction control, it is possible to sufficiently satisfy both the responsiveness ensuring the target air-fuel ratio and the securing of a sufficient air amount in the high load region.

The figure which shows an example of the engine to which this invention was applied. The figure which shows the example of a setting of the execution area | region of DeNOx control. The time chart which shows the control content of this invention. The block diagram which shows the example of a control system of this invention. The flowchart which shows the example of control of this invention. The flowchart which shows another example of control of this invention.

  In FIG. 1, reference numeral 1 denotes an engine (engine body), which is an in-line four-cylinder automobile diesel engine in the embodiment. The engine 1 has a cylinder 2, a cylinder head 3, and a piston 4 as is known. An intake port 6 and an exhaust port 7 are opened with respect to the combustion chamber 5 formed above the piston 4. The intake port 6 is opened and closed by an intake valve 8, and the exhaust port 7 is opened and closed by an exhaust valve 9. A fuel injection valve 10 is attached to the cylinder head 3 so as to face the combustion chamber 5. In the embodiment, the fuel injection is of a common rail type, and extremely high pressure fuel is injected from the fuel injection valve 10.

  In the intake passage 20 connected to the intake port 6, the air cleaner 21, the compressor wheel 22 a of the large exhaust turbocharger 22, and the compressor of the small exhaust turbocharger 23 are sequentially arranged from the upstream side to the downstream side. A wheel 23a, an intercooler 24, an air amount control valve (throttle valve) 25, and a surge tank 26 are provided. The surge tank 26 and each cylinder (the intake port 6 thereof) are independently connected by a branched intake passage 27.

  A bypass passage 28 is provided in the intake passage 20. The bypass passage 28 has an upstream end opened to the intake passage 20 between the compressor wheels 22a and 23a. Further, the downstream end of the bypass passage 28 is opened to the intake passage 20 between the compressor wheel 23 a and the intercooler 24. A control valve 29 is disposed in the bypass passage 28.

  In the exhaust passage 40 connected to the exhaust port 7, the turbine wheel 23b of the exhaust turbocharger 23, the turbine wheel 22b of the exhaust turbocharger 22, the NOx catalyst 41, A DPF 42 is connected.

  A waste gate passage 40a that bypasses the turbine wheel 22b is formed in the exhaust passage 40, and a waste gate valve 43 is disposed in the waste gate passage 40a. Further, a bypass passage 40b that bypasses the turbine wheel 23b is formed in the exhaust passage 40, and a control valve 40b is disposed in the bypass passage 40b. The downstream end of the bypass passage 40b is opened between the exhaust passage 40 and the downstream end of the turbine wheel 23b and the upstream end of the bypass passage 40a.

  The intake passage 20 and the exhaust passage 40 are connected via an EGR passage 50. The upstream end of the EGR passage 50 is opened to the exhaust passage 40 on the upstream side of the turbine wheel 23b. Further, the downstream end of the EGR passage 50 is opened between the intercooler 24 and the air amount control valve 25 in the intake passage 20.

  An EGR cooler 51 is connected to the EGR passage 50, and an EGR valve 52 is disposed on the downstream side of the EGR cooler 51. The EGR passage 50 is provided with a bypass passage 53 that bypasses the EGR cooler 51. The bypass passage 53 has an upstream end opened to the EGR passage 50 upstream of the EGR cooler 51, and a downstream end opened to the EGR passage 50 downstream of the EGR valve 52. An EGR valve 54 is disposed in the bypass passage 53.

  Next, an area where NOx reduction control (DeNOx control) is executed, that is, a reduction execution area will be described with reference to FIG. Note that the NOx reduction control is performed in the hatched region in FIG. 2 using the engine speed and the engine load as parameters. This DeNOx control execution region is also a region where supercharging is performed. The supercharging is performed mainly by the small exhaust turbocharger 23 at the time of low engine rotation (the control valve 29 is closed at this time), and by the large exhaust turbocharger 22 at the time of high engine rotation. Supercharging is performed (at this time, the control valve 29 is opened).

  The engine load range set in the reduction execution region shown in FIG. 2 is divided into a low load region and a high load region in advance with a predetermined load as a boundary. In the low load region, the air amount adjustment when the air-fuel ratio of the exhaust gas is set to the target air-fuel ratio necessary for NOx reduction is performed by adjusting the opening amount of the air amount control valve 25. On the other hand, in the high load region, the air amount adjustment when the air-fuel ratio of the exhaust gas is set to the target air-fuel ratio necessary for NOx reduction is performed by adjusting the air amount by adjusting the supercharging state.

  The reduction execution region is a region where supercharging by the exhaust turbocharger 23 is mainly performed. For this reason, the air amount adjustment by the supercharging pressure control as the supercharging state is performed by adjusting the opening degree of the control valve 29. It is possible to adjust the air amount by adjusting the supercharging pressure by adjusting the opening of the control valve 44, but it is preferable to adjust the opening of the control valve 29 from the viewpoint of responsiveness.

  As execution conditions for executing DeNOx control, the NOx occlusion amount in the NOx catalyst 41 is greater than or equal to a predetermined upper limit value, is in the reduction execution region shown in FIG. 2, and the engine coolant temperature is greater than or equal to a predetermined temperature. The exhaust gas temperature is set to satisfy all the conditions that the exhaust gas temperature is equal to or higher than the predetermined temperature and the NOx catalyst 41 is equal to or higher than the predetermined temperature. However, the present invention is not limited to this, and the execution conditions can be set as appropriate.

  The DeNOx control is basically started when the NOx occlusion amount in the NOx catalyst 41 becomes equal to or higher than a predetermined upper limit value and is performed until the NOx occlusion amount in the NOx catalyst 41 becomes zero. However, the DeNOx control may be stopped in a state where the NOx occlusion amount in the NOx catalyst 41 is reduced to a lower limit value greater than zero. The detection (or estimation) of the NOx occlusion amount in the NOx catalyst 41 can be performed by a known appropriate method. For example, a sensor that detects the occluded NOx amount is used, or data indicating the engine driver state is used. And NOx occlusion amount can be estimated.

  In the case of DeNOx control, post injection is performed in addition to main injection for obtaining necessary torque. This post-injection is performed during the expansion stroke, more specifically in the first half of the expansion stroke, so as to be burned in the cylinder in order to suppress discharge of unburned fuel from the cylinder and oil dilution suppression. In addition, EGR is executed in order to prevent or suppress the occurrence of soot during DeNOx control (the EGR valve opening is kept constant). The exhaust gas recirculation by EGR execution delays the ignition of the post-injected fuel and ensures the time until the post-injected fuel is combusted (the post-injected fuel is suitable for air and fuel) Ignited in a mixed state).

  Next, referring to the time chart of FIG. 3, for DeNOx control, in particular, air amount adjustment by the air amount control valve 25 and supercharging pressure adjustment by the control valve 29 (air amount adjustment accompanying supercharging pressure adjustment). Focus on the explanation. In FIG. 3, when the DeNOx execution flag is “0”, it indicates that the execution conditions for the DeNOx control are not satisfied, and when it is “1”, the execution conditions for the DeNOx control are satisfied. Further, when the flag of the throttle FB region determination is “1”, it indicates that the load is low in the reduction execution region shown in FIG. 2, and when it is “0”, the flag is high in the reduction execution region shown in FIG. Indicates a load area.

  In the embodiment, the target air-fuel ratio required for NOx reduction is set to 0.96, which is smaller than 1 where the excess air ratio λ becomes the stoichiometric air-fuel ratio (becomes the rich side). The supercharging pressure is a target supercharging pressure that is set according to the engine operating state (feedback control).

  First, until just before time t1, NOx reduction control is not being performed. At time t1, the execution condition of the NOx reduction control is satisfied, and the low-load area is in the reduction execution area. At this time, first, the opening degree of the air amount control valve 25 is decreased in order to enrich the air-fuel ratio. The supercharging pressure is controlled so as to become the target supercharging pressure in accordance with the operating state of the engine. At the time t2, the opening degree of the air amount control valve 25 is minimized, and the post injection is started from this point and the post injection amount is gradually increased.

  At time t3, the air-fuel ratio of the exhaust gas has converged near the target air-fuel ratio. After t3, the supercharging pressure converges to the target supercharging pressure, and the opening amount of the air amount control valve 25 is increased accordingly. Since the supercharging pressure converges to the target supercharging pressure and the air-fuel ratio of the exhaust gas also converges to the target air-fuel ratio, post injection is maintained at a substantially constant amount.

  At time t4, the throttle FB region determination flag is set to “1”, and becomes the high load region in the reduction execution region. After the time point t4, the air-fuel ratio is converged to the vicinity of the target air-fuel ratio. Therefore, the supercharging pressure is controlled by adjusting the air amount by the supercharging control valve 29 so that the air-fuel ratio becomes the target air-fuel ratio (feedback). control). Further, the air amount control valve 25 is fully opened.

  FIG. 4 shows an example of a control system for performing DeNOx control. In the figure, U is a controller (control unit) configured using a microcomputer. The controller U receives at least signals from various sensors S1 to SS6. S1 is an intake air amount sensor that detects the intake air amount, and is disposed, for example, on the downstream side of the air cleaner 21 in the intake passage 20. S2 is a rotation speed sensor that detects the engine rotation speed. S3 is a load sensor that detects an engine load (accelerator opening).

  S4 is an air-fuel ratio sensor (oxygen sensor) that detects the air-fuel ratio of the exhaust gas. In the embodiment, S4 is disposed on the downstream side of the DPF 42 in the exhaust passage 40. It can also be arranged immediately upstream. S5 is a temperature sensor that detects the exhaust gas temperature, and is disposed, for example, upstream of the opening position of the EGR passage 50 in the exhaust passage 40. S6 is a supercharging pressure sensor, which is disposed in the surge tank 26, for example. The controller U controls the EGR valves 52 and 54 in addition to the fuel injection valve 10 and the air amount control valve 25. The target air-fuel ratio at the time of DeNOx control is feedback-controlled so that λ (excess air ratio) is set to, for example, 0.96, and becomes the target air-fuel ratio (range) using the air-fuel ratio sensor S4. The In the feedback control, if the air-fuel ratio of the exhaust gas is in the range of λ = 0.96, which is the target air-fuel ratio, and λ = 1, which is the theoretical air-fuel ratio, the allowable range is assumed. Note that the reduction execution area as shown in FIG. 2 is stored in advance in the controller U (memory thereof).

  Next, the control contents of the controller U will be described with reference to the flowchart of FIG. In the following description, Q indicates a step. First, in Q1, based on the current engine speed and engine load, it is determined whether or not it is the reduction execution region shown in FIG. If YES in Q1, the execution flag for DeNOx control is set to 1 in Q2.

  After Q2, in Q3, it is determined whether or not the reduction execution region shown in FIG. 2 is a low load region. If YES in Q3, feedback control is performed in Q4 so that the air amount is reduced by controlling the air amount control valve 25 in the valve closing direction so that the air-fuel ratio of the exhaust gas becomes the target air-fuel ratio. The

  After Q3, at Q4, the opening degree of the control valve 29 is controlled so that the target boost pressure is set according to the engine operating state (supercharging pressure control). Thereafter, in Q5, in addition to the main injection, post injection is executed.

  After Q6, it is determined in Q7 whether or not the NOx occlusion amount in the NOx catalyst 41 has been reduced to almost zero. If the determination in Q7 is NO, the process returns to Q3. If the determination in Q7 is YES, the NOx reduction control is completed, and the process returns after the DeNOx execution flag is reset to 0 in Q8.

  When the determination in Q3 is NO, it is a high load region in the reduction execution region shown in FIG. At this time, it is determined in Q9 whether or not the boost pressure has converged near the target boost pressure and the air-fuel ratio of the exhaust gas has converged near the target air-fuel ratio. If the determination in Q9 is NO, the process proceeds to Q4 (a state of waiting for the supercharging pressure to converge near the target supercharging pressure and the air-fuel ratio of the exhaust gas to converge near the target air-fuel ratio).

  If YES in Q9, the air amount control valve 25 is fully opened in Q10. Thereafter, in Q11, feedback control is performed so that the air-fuel ratio of the exhaust gas becomes the target air-fuel ratio by the supercharging pressure control by controlling the control valve 29. Thereafter, at Q12, main injection and post injection are performed.

  After Q12, at Q13, it is determined whether or not a supercharging pressure overshoot has occurred. If the determination in Q13 is NO, the process proceeds to Q7. If YES in Q13, the air amount control valve 25 is controlled in the valve closing direction in Q14 to reduce the air amount.

  After Q14, it is determined in Q15 whether or not the air-fuel ratio of the exhaust gas is in a range between the target air-fuel ratio and the stoichiometric air-fuel ratio. If the determination in Q15 is NO, the process proceeds to Q7. Further, when the determination in Q15 is YES, the post injection amount is decreased.

  When the determination in Q1 is NO, the DeNOx control is not executed. At this time, only the main injection is performed in Q17 (no post injection).

  FIG. 6 shows another control example of the present invention. In the control example of FIG. 5, control is performed by adding the process of Q21 between Q3 and Q9. The control content shown in FIG. 6 is processing in which the DeNOx control is always started from the low load region and is not started from the high load region. That is, when DeNOx control is started by air amount control by supercharging pressure control, the response delay for achieving the target air-fuel ratio becomes considerably large. For this reason, the DeNOx control is started from the air amount control by the air amount control valve 25 with a very small response delay.

  Specifically, when the determination in Q3 in FIG. 6 is NO (in the high load region), it is determined in Q21 whether or not the DeNOx control is started. The determination at Q21 is a determination as to whether or not the DeNOx control is started from the high load region (in other words, the transition from the state where the DeNOx control is performed in the low load region). It also determines whether or not there is). If YES in Q21, the process returns as it is to prohibit DeNOx control. Further, when the determination of Q21 is NO, the processing after Q9 in FIG. 5 is performed. The determination of Q21 can also be made as to whether or not DeNOx control in the low load region has been continuously performed for a predetermined time or more.

  Although the embodiment has been described above, the present invention is not limited to the embodiment, and can be appropriately changed within the scope described in the scope of claims. For example, the invention includes the following cases. . The engine 1 may be a gasoline engine. Only one exhaust turbocharger may be provided, and a variable vane type may be used instead of a waste gate valve type. Each step or step group shown in the flowchart indicates the function of the controller U, and the name indicating the function can be attached to the name of the means so as to be grasped as a constituent requirement of the controller U. In feedback control when the air-fuel ratio of the exhaust gas is set to the target air-fuel ratio, the range between the theoretical air-fuel ratio and the target air-fuel ratio richer than that is not set as an allowable range, but the target air-fuel ratio is set as a boundary. It may be performed on or off. Of course, the object of the present invention is not limited to what is explicitly stated, but also implicitly includes providing what is considered preferable or substantially preferable, and can also be understood as a control method. It is.

  The present invention is suitable for application to, for example, an automobile diesel engine.

U: Controller S2: Revolution sensor S3: Load sensor S4: Air-fuel ratio sensor S5: Temperature sensor (for detecting exhaust gas temperature)
S6: Supercharging pressure sensor 1: Engine 2: Cylinder
DESCRIPTION OF SYMBOLS 10: Fuel injection valve 20: Intake passage 22: Exhaust turbo type supercharger 22a: Compressor wheel 22b: Turbine wheel 23: Exhaust turbo type supercharger 23a: Compressor wheel 23b: Turbine wheel 25: Air quantity control valve 40: Exhaust Passage 41: NOx catalyst 50: EGR passage 52: EGR valve 54: EGR valve

Claims (4)

A fuel injection valve for supplying fuel to the engine;
An air amount control valve for adjusting the amount of air supplied to the engine;
An exhaust turbocharger,
A supercharging control valve for adjusting a supercharging state by the exhaust turbocharger;
NOx in the exhaust gas is occluded when the exhaust gas air-fuel ratio is leaner than the stoichiometric air-fuel ratio, and the exhaust gas air-fuel ratio is near the stoichiometric air-fuel ratio or higher than the stoichiometric air-fuel ratio. A NOx catalyst that reduces the stored NOx when it is rich,
When the NOx reduction condition is satisfied, the NOx reduction control for reducing the NOx stored in the NOx catalyst by setting the air-fuel ratio of the exhaust gas to a target air-fuel ratio capable of reducing the NOx stored in the NOx catalyst is executed. NOx reduction control means for
With
The NOx reduction control means adjusts the air amount by the air amount control valve and the supercharging control valve and sets the air-fuel ratio of the exhaust gas to the target air-fuel ratio, and adjusts the air amount by the air amount control valve Is set to allow the air amount adjustment by the supercharging control valve after the air-fuel ratio of the exhaust gas is converged to the vicinity of the target air-fuel ratio by
An exhaust emission control device for an engine. In claim 1,
The NOx reduction condition is set when the engine operating state is at least in the reduction execution region set with the engine load as a parameter, and the reduction execution region is divided into a low load region and a high load region in advance. And
The NOx reduction control means adjusts the air amount by the air amount control valve when in the low load region of the reduction execution region, while the engine load is in the high load region of the reduction execution region. Adjusts the air amount by the supercharging control valve to control the air-fuel ratio of the exhaust gas to the target air-fuel ratio, and the air-fuel ratio of the exhaust gas is controlled by the air amount control by the air amount control valve. It is set to allow the air amount adjustment by the supercharging control valve after being converged near the target air-fuel ratio,
An exhaust emission control device for an engine. A fuel injection valve for supplying fuel to the engine;
An air amount control valve for adjusting the amount of air supplied to the engine;
An exhaust turbocharger,
A supercharging control valve for adjusting a supercharging state by the exhaust turbocharger;
NOx in the exhaust gas is occluded when the exhaust gas air-fuel ratio is leaner than the stoichiometric air-fuel ratio, and the exhaust gas air-fuel ratio is near the stoichiometric air-fuel ratio or higher than the stoichiometric air-fuel ratio. A NOx catalyst that reduces the stored NOx when it is rich,
When the NOx reduction condition is satisfied, the NOx reduction control for reducing the NOx stored in the NOx catalyst by setting the air-fuel ratio of the exhaust gas to a target air-fuel ratio capable of reducing the NOx stored in the NOx catalyst is executed. NOx reduction control means for
With
The NOx reduction condition is set when the engine operating state is at least in the reduction execution region set with the engine load as a parameter, and the reduction execution region is divided into a low load region and a high load region in advance. And
The NOx reduction control means adjusts the air amount by the air amount control valve when the engine load is in the low load region of the reduction execution region, while the engine load is in the high load region of the reduction execution region. In this case, the air amount adjustment by the supercharging control valve is performed, and the control of the air amount adjustment by the supercharging control valve is performed from the state in which the air amount adjustment by the air amount control valve has been performed. Set to allow at the time of transition,
An exhaust emission control device for an engine. In claim 3,
The NOx reduction control means controls the air amount adjustment by the supercharging control valve on the condition that the air-fuel ratio of the exhaust gas has converged near the target air-fuel ratio by the air amount adjustment by the air amount control valve. An exhaust emission control device for an engine characterized by being set to allow.

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